We're full-timers and have been for 7 years now. I don't think it's how many miles per year as much as how long you stay per stop. If you plan on staying in one spot for many months, the engine (and such) don't get exercised enough, therefore a 5th wheel might fit your plans better. If, however, you move more often, say every week or two weeks, a motor home makes more sense. As far as gas vs diesel, that depends on other items, where you travel (mountains or flat), how much you carry and such. DPs are for sure heavier and I think ride/handle better than gas models. No one size fits all here.
Thanks Chuck